1. Field of the Invention
The present invention relates generally to an Orthogonal Frequency Division Multiple Access (OFDMA) communication system, and in particular, to a system and method for managing a band AMC (Adaptive Modulation and Coding) channel in a communication system using a Time Division Duplex (TDD)-based OFDMA scheme (hereinafter referred to as a “TDD-OFDMA communication system”).
2. Description of the Related Art
Research into a 4th generation (4G) communication system, which is the next generation communication system, is currently being conducted to provide users with differing services having various Qualities of Service (QoSs) and supporting a data rate of about 100 Mbps. Compared with the 4G communication system, a 3rd generation (3G) communication system generally supports a data rate of about 384 Kbps in an outdoor channel environment having poorer channel conditions, and supports a data rate of a maximum of 2 Mbps in an indoor channel environment having better channel conditions.
A wireless Local Area Network (LAN) system and a wireless Metropolitan Area Network (MAN) system generally support a data rate of 20 to 50 Mbps. Currently, therefore, active research into the 4G communication system is being carried out to develop a new communication system guaranteeing mobility and QoS in the wireless LAN system, and in order to develop the wireless MAN system guaranteeing a higher data rate, in order to support a high-speed service provided in the 4G communication system.
Accordingly, a great deal of research is being conducted on an Orthogonal Frequency Division Multiplexing (OFDM) scheme for high-speed data transmission through wired/wireless channels in the 4G communication system. The OFDM scheme is to a scheme of transmitting data using multiple carriers, and is a kind of a Multi-Carrier Modulation (MCM) scheme, which parallel-converts a serial input symbol stream into parallel symbols and modulates the parallel symbols with a plurality of orthogonal subcarriers, i.e., a plurality of subcarrier channels, before transmission.
A multiple access scheme based on the OFDM scheme is the OFDMA scheme. In the OFDMA scheme, subcarriers in one OFDMA symbol are divided for a plurality of users, i.e., subscriber stations (SSs). Communication systems using the OFDM/OFDMA scheme include an Institute of Electrical and Electronics Engineers (IEEE) 802.16a communication system, an IEEE 802.16d communication system, and an IEEE 802.16e communication system. The IEEE 802.16d communication system is a system to which the OFDMA scheme is applied to support a broadband transmission network to a physical channel for the wireless MAN system. Further, the IEEE 802.16d communication system is a Broadband Wireless Access (BWA) communication system using a TDD-OFDMA scheme. Therefore, the IEEE 802.16d communication system, in which the OFDM/OFDMA scheme is applied to the wireless MAN system, transmits a physical channel signal using a plurality of subcarriers, thereby enabling high-speed, high-quality data transmission.
FIG. 1 is a diagram schematically illustrating a frame structure used in a conventional TDD-OFDMA communication system. Referring to FIG. 1, a frame used in the TDD-OFDMA scheme is divided into a downlink (DL) interval 149 and an uplink (UL) interval 153. A Transmit/receive Transition Gap (TTG) 151 is formed in an interval where transition occurs from the DL 149 to the UL 153, as a guard time, and a Receive/transmit Transition Gap (RTG) is formed in an interval where transition occurs from the UL 153 back to the DL 149, as a guard time. The TDD-OFDMA frame has a vertical axis including a plurality of subchannels 147 and a horizontal axis including a plurality of OFDMA symbols 145.
Describing the DL 149, a preamble 111 for synchronization acquisition is located in a kth OFDMA symbol, and broadcast data information that SSs will receive in common, such as a frame control header (FCH) 113, DL-MAP 115, and UL-MAP 117, is located in a (K+1)th or (K+2)th OFDMA symbol. The FCH 113 includes two subchannels, and transmits basic information on subchannels, i.e., raging and modulation schemes. DL bursts 121, 123, 125, 127, and 129 are located between the (K+2)th OFMDA symbol exclusive of a UL-MAP 119 and a (K+8)th OFDMA symbol.
Describing the UL 153, preambles 131, 133, and 135 are located in a (K+9)th OFDMA symbol, and UL bursts 137, 139, and 141 are located between a (K+10)th OFDMA symbol and a (K+12)th OFDMA symbol. In addition, a ranging subchannel 143 is located between the (K+9)th OFDMA symbol and the (K+12)th OFDMA symbol.
Information on positions and allocation of the UL bursts 137, 139, and 141 and the DL bursts 121, 123, 125, 127, and 129 is provided from a base station (BS) controlling a particular cell to SSs, which are located in the cell, through the DL-MAP 115 and the UL-MAP 117. The SSs are variably allocated subchannels, each of which a combination of frequencies and symbols, through the information every frame, and perform communication using the allocated subchannels. That is, the SSs can use different subchannels every frame, instead of fixed subchannels. Also, in a neighbor cell, SSs perform communication using the same frequency band. Therefore, for an SS located in a cell boundary, if different cells use the same subchannels, the subchannels may significantly interfere with each other.
As described above, the conventional OFDMA communication system performs data communication with SSs regardless of channel states. That is, a scheme that enables SSs having good channel states to perform high-speed, high-capacity communication is not presented in the frame structure for the conventional OFDMA communication system. Therefore, the conventional technology cannot perform a flexible modulation and coding method for the SSs having high subchannel quality.